1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices, and more specifically to a method of fabricating semiconductor devices with contact hole structures formed in insulating films for electrically connecting electrodes constructed of refractory metals or other like conductive material to diffusion layers provided in semiconductor substrates.
2. Conventional Art
FIG. 6 is a cross sectional view illustrative of an example of a conventional method of forming electrodes via contact holes. As illustrated in FIG. 6(a), a LOCOS oxide film 22 is formed on a silicon substrate 21 by conventional lithography and selective oxidation. Ions of boron difluoride are then implanted at 70 KeV with a dose of 5.times.10.sup.15 cm.sup.-2, and this is followed by activation treatment (annealing) at 900.degree. C. to form a conductor region 23. Such conductor regions are often called diffusion regions as well. An interlayer dielectric film 24 is formed on the conductor region 23 to a height of 1.5 .mu.m, for example, and a contact hole is formed by conventional photolithography using an i-line exposure combined with dry etching. When the contact hole is formed, a native oxide film 23a is formed on the surface of the conductor region 23 and elsewhere as illustrated in FIG. 6(a). This native oxide film 23a is then etched off to a depth of 5 nm, for example, with 1 (vol.) % hydrofluoric acid or the like, thus substantially removing the oxide 23a as illustrated in FIG. 6(b).
Thereafter, as illustrated in FIG. 6(c), titanium (Ti) 25 is deposited as the contact metal for forming an ohmic contact on the bottom of the contact hole by sputtering in a vacuum or by Chemical Vapor Deposition (LPCVD), to a height of 10 nm. Then, a 50 nm-thick film of titanium nitride 26 is formed on the titanium 25 to improve the barrier properties against the aluminum wiring to be formed thereon. Then, as illustrated in FIG. 6(d), aluminum 27 is formed thereon by sputtering, and each of the foregoing films is patterned by lithography as required. Here, as a preparation to wiring, the hole may be filled with tungsten (W) metal prior to the formation of the aluminum 27 especially when the contact hole has a high aspect ratio.
With the fabricating method of this conventional art, however, when the contact hole diameter is decreased with the downsizing of the device elements, and with the increase in integration, pretreatment with diluted hydrofluoric acid before the formation of a contact metal after the contact hole has been opened poses a problem. That is, since this diluted hydrofluoric acid etching is an isotropic etching with a low etching selectivity ratio of the native oxide film 23a to the interlayer dielectric film 24 made of silicon oxide or the like, the portion of the interlayer dielectric film 24 which forms the sidewall of the contact hole is undesirably etched off, as indicated by broken lines in FIG. 6(a), thus presenting the problem of undesirably large aperture. This not only prevents the pursuit of downsizing and integration, but also causes wiring shortage and junction leakage, resulting in a lower degree of device reliability.
In addition, according to the method of the conventional art, in cases where the conductor region is a P.sup.+ diffusion layer, the native oxide film and/or the etching residue on the surface thereof cannot be removed to a satisfactory degree, and this presents a problem in that the contact resistance of the P.sup.+ diffusion layer cannot be as low as that of the N.sup.+ diffusion layer. This problem becomes more significant as the bottom of the contact has a smaller area and as the PN junction has a shallower depth, thus lowering the reliability of the device and preventing provision of higher-speed circuits.
In order to overcome such problems, it has been proposed to add pretreatment with a plasma of argon, hydrogen or a mixture of these gases just before the formation of the electrode. Results of investigations on contact resistivity characteristics by use of electron-cyclotron-resonance plasma enhanced chemical vapor deposition (ECR-CVD) are reported in the Technical Report of the IEBICE (the Institute of Electronics, Information and Communication Engineers), Vol. 92, No. 344, 61-66, 1992 (published in Japanese). According to the report, an N.sup.+ or P.sup.+ diffusion layer is formed on a silicon substrate, and a BPSG film is deposited on the diffusion layer, a contact hole with an aperture of 0.5 to 1.0 .mu.m and an aspect ratio of 1.7 to 4.0 is then formed in the BPSG film. The substrate is then treated with diluted hydrofluoric acid in order to remove the native oxide film, followed by exposure to ECR plasma of argon and hydrogen, which is then followed by ECR-CVD to form a 30 nm-thick Ti film and a 100 nm-thick TiN film. Here, the substrate is heated to 420.degree. C., and the reaction pressure is 1 mTorr. After the Ti and TiN films are formed, the substrate is subjected to thermal treatment in an atmosphere of nitrogen at 760.degree. C. for 30 seconds, the contact hole is then buried by blanket tungsten CVD and a successive etch back process. Finally, aluminum wiring is formed, and the contact resistance was measured. According to the reported results, contact resistances on the N.sup.+ diffusion layer and on the P.sup.+ diffusion layer are not very different from each other when the aperture of the contact is as large as 0.7 .mu.m, whereas the N.sup.+ contact and the P.sup.+ contact have apparently different resistances when the aperture of the contact is 0.6 .mu.m or smaller.
Close examinations made by the present inventors on much more fine dimension contact holes with depths of 2.1 .mu.m and apertures of up to 0.35 .mu.m have revealed that the N.sup.+ contact and the P.sup.+ contact have increasingly different resistances as the aperture width decreases, as shown in FIG. 7. Further study by the present inventors has revealed that according to the reported method, only a same degree of effect as when the treatment is only with hydrofluoric acid was being produced, as shown in FIG. 8.
Recently, it has been suggested to add an Ar ECR plasma pretreatment to the wet treatment using hydrofluoric acid. Referring to abstract number 29p-K-18 (page 736) of the Extended Abstracts of the 42nd Spring Meeting (1995) of the Japan Society of Applied Physics and Related Societies, for example, the use of Ar ECR plasma results in a lower resistance on the P.sup.+ diffusion layer than according to the wet treatment. However, the contact resistance value is still a few times larger than that of the value on the N.sup.+ diffusion layer, and thus the reported method cannot prevent impairment of the device characteristics.
As mentioned above, according to the conventional method using wet pretreatment, since the etching is isotropic and etching selectivity ratio of the native oxide film on the bottom of the contact hole to the interlayer dielectric film made of silicon oxide or the like is low, the etching of the native oxide film leads to etching of the sidewall of the contact hole as well, thus increasing the aperture, as illustrated in FIG. 6. This tends not only to prevent the downsizing and the integration, but also to cause wire shorts and leakage through the junction, and thus the reliability is impaired. Also, since all the reported methods fail to satisfactorily remove the native oxide film, etching residue, etc. on the P.sup.+ diffusion layer, there is presented a problem in that the contact resistance on the P.sup.+ diffusion layer cannot be as low as on the N.sup.+ diffusion layer. This can be another reason for poorer electric characteristics of the device.